What Is Winter Solstice? Learn About the Shortest Day of the Year

Thursday in the northern hemisphere marks the winter solstice, the single day that has the shortest period of daylight of the year. The day is also the summer solstice, or longest day of the year, in the southern hemisphere.

The solstices, and the seasons they mark, are the result of a celestial coincidence. Earth's days are caused by the planet spinning around its axis, the invisible line that would connect its north and south poles. At the same time, the planet is traveling on a yearly stroll around the sun. But the axis isn't quite perpendicular to the path of its orbit, instead it is tilted on its side about 23 degrees, with the axis always pointing the same way into space.

That means that for half the year, the north pole is tilted a bit toward the sun (and experiences summer) and that for the other half of the year, the south pole is tilted a bit toward the sun, giving the northern hemisphere winter. The solstice, the precise time at which the axis and the sun align, marks the halfway point of this period. That time this year is at 4:28 PM Universal Time, or 9:28 PM Eastern time on Thursday.

The solstice also marks the day with the fewest hours of sunlight in the northern hemisphere, giving the solstice the nickname the shortest day of the year. Precisely how short that is varies with how far north or south you are—around the solstice, the sun never truly rises at the north pole.

While the shortest day can be kind of a bummer, it has a little silver lining: It means that from here, sunlight will stick around longer and longer. And that's why, for millennia, humans have celebrated the solstice—for the return of the sun that it marks.

Senator Bob Menendez said he was "disappointed, but not surprised, that the Trump administration has failed once again to prioritize our long-term national security interests or stand up for human rights."